Recently, successful studies of a fiber laser generating a short pulse through passively mode-locking using a carbon nanotube saturable absorber have been reported in various papers and Patent Documents. However, according to the related art up to now, a technology of actively, easily, and variously changing properties of the pulse with respect to the fiber laser has not existed, such that many efforts to change the pulse have been required.
As an example of the related art, “Passively Mode-locked Fiber Laser Using Carbon Nanotubes” has been disclosed in US Patent Laid-Open Publication No. 2010-0296527A1. In this case, a repetition rate of a pulse may be changed by controlling a cavity length. However, since the entire cavity is made of fibers, it may be impossible to actively control the length. Therefore, the cavity length is passively controlled by replacing some of the optical components such as the fiber, or the like, such that the repetition rate may be changed from 316 to 415 MHz.
A system of stabilizing a repetition rate of the pulse laser using a principle similar to that in the above mentioned Patent Document has been reported in a paper entitled “Highly Stable, Frequency-Controlled Mode-Locked Erbium Fiber Laser Comb” (J. L. Peng, H. Ahn, R. H. Shu, H. C. Chui and J. W. Nicholson, Applied Physics B: Lasers and Optics, 86, 49-53, 2007). A repetition rate may be controlled by actively and finely controlling a cavity length using a piezoelectric device (PZT) mounted at an end portion of a fiber. However, in the system as described above, since the entire cavity is not made of only the fibers but includes optical components such as a condensing lens, or the like, a section at which the pulse is outside of the fiber to propagate in the air is generated, which causes the system to be unstable.
In addition, the method of generating a second harmonic wave using a KTP crystal device having secondary nonlinearity for multiple pulsing has been disclosed in a paper entitled “Multiple Pulsing and Harmonic Mode-Locking in an All-Normal-Dispersion Nd:GdVO4 Laser Using a Nonlinear Mirror” (J. H. Lin and K. H. Lin, J. Phys. B: At. Mol. Opt. Phys. Vol. 43, 065402, 2010). The generated second harmonic wave causes group velocity mismatch with an original fundamental wave, but third, fourth, fifth harmonic waves are generated by actively controlling a distance between the KTP crystal and an output coupler. However, in this method, since a secondary non-linear device such as the KTP crystal is required and a position thereof should be controlled, it may be impossible to use this method in a system entirely based on the fiber.